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Aviation History
1959
1959 - 0601.PDF
298 FLIGHT Programme-controlled Altitude Chamber Complete Flight- plans Simulated in D.H. Propellers' NewTest Facility By P. R. PRIOR* Interior of the chamber MOST manufacturers of aircraft accessories are finding itincreasingly essential to be able to simulate on the groundconditions appropriate to flight at high speeds and high altitudes. As manufacturers of major air-conditioning components,de Havilland Propellers Ltd. have spent a considerable sum on an advanced altitude chamber in which any desired condition canbe achieved, with the individual parameters separately variable by an automatic control system. D.H. Propellers delegated to Honey-well Controls Ltd. the responsibility for the design and develop- ment of an automatic control system which can automaticallysimulate a complete mission. Pneumatic control was chosen for fast response. After nine months of co-operation between the two companiesthe chamber was built and tested. Careful dovetailing of chamber results with those obtained in flight established the validity of themethod. From this stage progress was rapid. Shortcomings of prototypeequipment were quickly detected. The air-conditioning unit for one aircraft, for instance, had passed all its official sea-level andaltitude tests at quite severe and apparently representative con- ditions when unexpectedly great difficulties arose in service. Underthe representative conditions produced by the altitude plant it was found that the original tests had failed to show certain short-comings which were now immediately apparent. A rapid pro- gramme, checked at every stage by reproducing identical flightconditions, established within a very short time where the fault lay and showed the design team exactly which of their moveswould have the greatest effect. Following final remedial action no further trouble was encountered. It was clear that a test facilityhad been created in which research data could be compiled more cheaply, more conveniently, and in many cases more accurately,than in an aircraft during flight. Consisting of seven sets of programme controllers, the controlsystem regulates the seven most important variables affecting air- conditioning units during flight. On each controller a pointerindicating ihe required value for the variable bears Upon the profile of a programming disc, the contours of which cause the pointer tomove in accordance with a predetermined flight plan. The con- troller interprets these movements of thepointer and changes the variable accord- ingly. The addition of derivative action tosome of the controllers provides a system of control which is in theory infinitelyflexible and which can call for rates of change as high as any of those that can beexpected to occur in actual flight. In prac- tice the severity of the conditions is limitedonly by the power available. Variab'es Controlled. The behaviour ofaircraft air-conditioning systems depends, of course, largely on speed and altitude,engine setting and ambient temperature and humidity. In the test plant the sevenmost important variables are regulated and synchronized by the programme controlsystem. Two of the controllers govern the temperature and pressure of the air simu- lating the cabin supply tapped from the engine compressors; tworegulate the air supplied to the secondary circuit; and the remain- ing three control the independent altitude pressures—cabinaltitude, secondary-circuit discharge and ambient altitude. The controllers regulate conditions in the 1,500 cu ft chamberby calling upon four sources of power—a high-pressure air supply operating up to 220 lb/sq in abs; a medium-pressure air supplyat 110 lb/sq in abs; heating and cooling capacity sufficient to produce air-supply temperatures ranging from — 50 to + 350 degC; and an extraction plant to simulate altitudes to 80,000ft. Pressure Control. The high-pressure supply is led from thecompressors through a control valve, heat exchanger and electric pyro-bar heater into the chamber, where it is connected to thesystem under test. Temperature and pressure pick-ups are in- stalled at the appropriate points and the measurements taken arecontinuously transmitted to the controllers concerned. As already briefly explained, the existing pressure at any moment is indicatedby a recording needle, the position of which coincides with the position of a second needle that follows the profile of the pro-gramming disc; and the controller, sensing any difference that may occur between desired and measured values, changes thepressure at the required rate by closing or opening the supply pressure control valve. The recording needle continues to recordinstantaneous system-pressures, and the completed chart con- firms the accuracy of control obtained. In this way, the supplypressure to the system is altered at the required rate and the correct system-pressure is maintained at any instant. Themedium-pressure air supply operates on a similar principle. Temperature Control. The programme controllers for tempera-ture operate a series of pneumatic switches. If, for example, the supply heaters are full on when a drop in temperature is calledfor by the controllers, the pneumatic output from the latter increases. This gradually releases the pneumatic sequence-switches and successive heater bars are taken out of circuit. By the time the controller's pneumatic output has reached half itsmaximum value all the heater bars are off. If this is not adequate to produce the required rate of fall, the control pressure risesfurther. As this occurs, an increasing quantity of pre-cooled high- pressure air is supplied to a slave refrigeration unit serving theheat exchanger in the main supply circuit. Forced cooling is thus supplied to the circuit until the demands of the programme aresatisfied. Sufficient reserve is available for temperature-changes as rapid as any that could be expected to occur in flight. Altitude Control. The controllers for altitude operate diaphragmvalves situated in pipes between the chamber and a second damp- ing tank. This tank is maintained at a certain ceiling altitude,which is always above that required for any of the test circuits. The controllers merely interpose pressure-drops between themain-chamber control-points and the damping tank, thus ensuring that the programmed altitude is always maintained. The whole programming cycle can be "frozen" at any instant,in which case the controllers continue to maintain the existing set of conditions. The response of each controller can be modulatedby means of variable proportional and integral action, and the whole cycle can be slowed down to operate at any required speed. *Honeywell Controls Ltd. 300 - Typical graph of system conditions for a giren flight plan. Fed with this information, the controllers cause the complete flight to be simulated in the altitude chamber -1OO TAXI i AND t TAKE- OFF I RETURN TO BASE SEC. AIR DISCHARGE ALT. (ft) SEC. AIR INLET TEMP CO ENG. TAPPING PRESS- URE (Ib/sq in obsj SEC. AIR INLETPRESSURE (Ib/sq in obsj •> 45,000 •^20000 TIME (min)
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